A resonant circuit (also referred to as a “resonant tank”) enables a resonant converter to change its gain (e.g., so as to compensate for changes to at its input and/or the requirements of a load) by adjusting the switching frequency of its power switches. A resonant converter may operate its power switches over a wide range of switching frequencies in order to achieve just the right output voltage or current at just the right time. To improve efficiency and reduce power losses, some resonant converters perform zero voltage switching (ZVS) or zero current switching (ZCS) techniques.
Some resonant converters rely either on a magnetizing current (e.g., in the case of an LLC converter) from the resonant circuit or a load current (e.g., in the case of a phase shift ZVS converter) to charge the output capacitance of their respective power switches so as to achieve ZVS or ZCS, within a reasonable amount of dead time. The level of the magnetizing current or load current depends on the amount of output capacitance associated with the power switches being charged during zero-voltage switch-on.
As is the case, some power switches may require a high magnetizing current to perform a ZVS operation. Use of a high magnetizing current can cause voltage transients, which have a high rate of change (dv/dt), to appear across the power switches. These voltage transients can cause damage and lead to unwanted capacitive couplings which can cause the resonant converter to transfer energy (e.g., from the primary to the secondary side of the resonant converter) when no energy should be transferred. Alternatively, some power switches will require a minimum amount of load current to perform a ZVS operation. The reliance on a load current to charge the output capacitance of its power switches may inhibit a phase shift ZVS converter from performing ZVS during light or no load conditions when the load current is less than the minimum amount being required.